The development of modem technology requires creating new materials that constitute a basis for fabricating optical, electronic, and other elements with desired properties. In particular, a necessary element in modern display design is an optically anisotropic film possessing an optimum combination of optical and mechanical properties for this application.
Optically anisotropic films may be fabricated using a variety of polymers. Anisotropic optical properties of the resultant films result from uniaxial extension, orientation in a given direction, modification with organic or inorganic (iodine) dyes, and the like. Poly(vinyl alcohol) (PVA) is employed as a base polymer (see for example Liquid Crystals: Applications and Uses, B. Bahadur, Ed., World Scientific, Singapore, 1990, Vol. 1, p. 101). However, because PVA-based films with dye additives have relatively low thermal stability, their application is limited.
Organic dichroic dyes may be used for the synthesis of optically anisotropic films with advantageous optical and working characteristics. Films based on these compounds may be obtained through application of a liquid-crystalline aqueous dye solution onto a substrate surface followed by evaporation of the solvent. Anisotropic properties may be imparted to the applied films through preliminary mechanical orientation of the substrate surface (see for example U.S. Pat. No. 2,553,961) or alternatively by means of an external aligning action, such as for example mechanical, electromagnetic, or other shearing forces, upon the film material in a liquid crystal state (see for example PCT patent publication No. WO 94/28073). The resulting dichroic films as well as polarizers and retarders based on these films, are characterized by desirable optical properties. However, these films nonetheless suffer from less than optimal physical, particularly mechanical, properties and are therefore not advantageously adapted for use under certain working and technological regimes.
For example, the liquid crystal display (LCD) technology involves the formation of protective layers, typically formed of thermostable polyimides obtained through cyclization of amidoacids at temperatures above 200° C. An alternative is offered by the use of a separately synthesized film possessing required parameters. Mechanical characteristics are important in applications of the optical anisotropic films such as protective masks, screens, spectacles, etc. Even simple sunglasses require materials combining excellent optical properties, radiation stability, chemical inertness with respect to moisture and sea salt, etc. Such materials are also employed in architecture, for example for antiglare coatings on windows and luminaries; for decorations such as stained glass and mosaic panels with wide color spectra that are stable under various weather conditions; and for other similar purposes.
The required working characteristics may be imparted to liquid crystal films using unique mechanical, hydrophobic, and thermal properties of macromolecular compounds-polymers. However, the formation of strong covalent bonds, which are responsible for these properties, by direct interaction of macromolecules with supramolecular dye complexes for thermodynamic reasons can lead to the irreversible degradation of supramolecules and, hence, to the impossibility of obtaining the final material possessing desired optical properties.
Other interactions, such as for example adhesion, between dyes and polymers traditionally employed as substrates, may introduce additional problems. Bonds formed upon the introduction of dyes into the structure of macromolecules by means of specific solvation, hydrogen bonds, hydrophobic interaction, or by other means are rather weak and the resulting systems are unstable. Films formed by these methods have characteristics that resemble those of to simple polymer—dye blends (see for example A. V. Tkachev, D. N. Kiselev, V. A. Tverskoi, and E. I. Soborover, “Polymethacrylates Containing Immobilized Dye: Optical and Sorption Properties,” Vysokomol. Soedin. 1994, 36 (8), 1326).
Another method for enhancing retention of a the dye phase in a polymer involves introducing dye molecules as dopants in the polymers, followed by liquid-crystalline aggregation of the dye on the polymer surface. Cyanine dyes have been employed as dopants in aromatic polyimides (see for example E. I. Mal'tsev, D. A. Lypenko, B. I. Shapiro, M. A. Brusentseva, E. V. Lunina, V. I. Berendyaev, B. V. Kotov, and A. V. Vannikov, Electroluminescent Properties of Aromatic Polyimides in the Presence of Organic Phosphors, Vysokomol. Soedin., Ser. A, 1999, 41 (9), 1480). However, the aggregation process is complicated, energy consuming, and does not provide for the obtaining of materials with desired optical properties.
U.S. Pat. No. 5,730,900 discloses a solution containing a discotic substituted polycyclic compound with polymerizable groups as substituents and a liquid-crystalline substance was applied onto an oriented polymer substrate, heated at a certain rate to about 100° C. to provide a homogeneous orientation of the discotic layer, irradiated with UV light at 110° C. for 20 min, and cooled to obtain a film comprising a polymer matrix with liquid-crystalline inclusions. However, the required individual selection of compatible solvents for the system components, and high temperatures and/or UV radiation required to initiate the polymerization process significantly limit the applicability of this method, especially for liquid-crystalline objects such as water-soluble dyes.
Accordingly, development of new methods and materials for the synthesis of optically anisotropic films that avoid these problems is desirable.